1. Field of the Invention
This invention relates generally to thermodilution catheters of the type that have an electric resistance-type heating element for applying heat to a patient's blood for purposes of measuring a physiological condition, such as volumetric blood flow. More specifically, the invention relates to systems and methods for maintaining the surface temperature of such a heating element at a level which will not be harmful to a patient.
2. Description of the Prior Art
Catheters have long been used for applying therapeutic or diagnostic preparations directly into the blood stream of animals or humans. Catheters are also commonly used to measure such parameters as cardiac output, blood pressure, blood volume, blood components and the like.
Numerous techniques have been disclosed in the prior art for measuring blood flow using catheters. Once such technique, termed indicator dilution, relies on the introduction of a marker into the bloodstream, the theory being that the marker will dissipate at a rate which is a function of blood flow as measured in units of volume per unit of time.
The present inventors believe, clinically, that heat is the preferred marker for such an indicator dilution system. Unlike other indicators, heat is conserved in the immediate vascular system, but is largely dissipated in the periphery in one circulation time so as to eliminate recirculation and accumulation problems. Cold (negative heat) is an indicator which can also be used very effectively in a clinical setting. Large amounts of cold may be used, for cold has relatively no deleterious effects on blood and surrounding tissues. However, a disadvantage of cold as an indicator is that it must be supplied in a chilled fluid carrier such as saline, because cold producing transducers are not commercially available. Cold-based indicator systems are disclosed in U.S. Pat. No. 4,819,655 to Webler and in U.S. Pat. No. 4,941,475 to Williams. Both of those systems have significant clinical limitations in that the circulating fluid must be cooled to near ice temperature prior to input into the catheter and temperature equilibrium must be established, which takes a significant amount of time. In addition, the enlarged catheter segment which is necessary for containing the cooling elements may restrict blood flow.
A disadvantage of heat as an indicator is that even small increases in heat transducer temperature can have a deleterious effect on blood and local tissue. In fact, it can be inferred from the teachings of Ham et al. in "Studies in Destruction of Red Blood Cells, Chapter IV Thermal Injury" Blood, Vol. 3, pp. 373-403 (1948), by Ponder in "Shape and Transformations of Heated Human Red Cells", J. Exp. Biol., Vol. 26, pp. 35-45 (1950) and by Williamson et al. in "The Influence of Temperature on Red Cell Deformability", Blood, Vol. 46, pp. 611-624 (1975), that a maximum safe filament surface temperature is probably about 48.degree. C.
A heater element in a catheter must satisfy several requirements if it is to be used clinically. Most importantly, the heat transducer or filament must be electrically safe. It also must only minimally increase the catheter cross-sectional area or diameter of the catheter and must be made of materials which are non-toxic and can be sterilized. Such a heater element must also be flexible so as not to increase the stiffness of the catheter body.
In prior art heat-type thermodilution catheters, either the heater element temperature is not monitored, or the temperature is measured with a second thermometer. Use of a second thermistor significantly adds to the cost of the catheter and provides a temperature measurement, but only at a single point. Accordingly, the measured temperature might not be representative of the surface temperature as a whole. Not monitoring heat or temperature does not allow for detection of undesirable events (e.g. low flow condition).
Gibbs, in an article entitled "A Thermoelectric Blood Flow Recorder in the Form of a Needle", Proc. Soc. Exp. Biol. & Med., Vol. 31, 1933, Pages 141-146, has suggested using the principle upon which a hot-wire anemometer operates to measure blood velocity. However, as noted by Gibbs in that article, such a technique has been limited to peripheral vessels and cannot give absolute blood volumetric flow rates, only velocity.
It is clear that there exists a long and unfilled need in the prior art for an improved system for maintaining the surface temperature of a thermodilution catheter heater element within safe physiological limits.